1. Field of the Invention
The present invention relates to a micropipette, a dispenser and a method for producing a biochip of DNA micro arrays or the like, more specifically, to a micropipette, a dispenser in which the micropipette is used, and a method for producing a biochip which are preferably used in the field of producing a biochip of DNA micro arrays or the like, in which case a work for arranging and fixing droplets having a very small volume on a substrate with a high density, that is, a work for forming micro spots, is required, so that a high precision work for forming micro spots is feasible and thus a high quality in products to be obtained can be attained.
2. Description of the Related Art
In recent years, a great progress is found in the method for analyzing a gene structure. In fact, various gene structures including the gene of a human kind have been made clear. In the analysis of such a gene structure, a DNA micro array is used, in which several thousands to several tens of thousands of different DNA fragments are arranged and then fixed as micro spots on a substrate such as a microscopic glass slide.
As a method for forming micro spots in producing a DNA micro array, the QUILL method, the pin and ring method or the spring-pin method is normally used. In all of the methods, it is necessary to suppress deviations in the volume and shape of each micro spot and to maintain the spacing between adjacent micro spots at a fixed value, and further to avoid the contamination due to the mutual mixture of the micro spots. A requirement for further increasing the density of the micro spots, further precisely forming the micro spots in a fine arrangement, and further enhancing the quality in a product to be obtained is still increasing.
In the QUILL method, a sample is retained at a concave part in the tip of a pin, and when the tip of the pin comes into contact with a substrate, the sample at the concave part is transferred to the substrate, and therefore a micro spot is formed there. In this method, there are the following problems: The reduction in the durability of the pin due to the deformation and/or a damage resulting from the contact of the tip of the pin with the substrate; and an increase in the cross contamination of the micro spots due to the incompleteness of cleaning the sample stored at the concave part.
In the pin and ring method, a sample solution in a micro plate is reserved by a ring, and the sample inside the ring is then picked up by pin passing through the inside of the ring in which the solution is reserved, thus enabling spots to be formed on a substrate. In this method, the number of samples, which can be reserved in one process, depends on the number of rings, and the latter number is normally less than ten. Accordingly, when it is necessary to form micro spots including several thousand to several tens of thousands of kinds of samples, several hundred to several thousand processes for cleaning and drying are required. This provides a problem regarding the productivity.
In the spring-pin method, however, a sample is adhered to the tip of a pin, and when the tip of the pin comes into contact with a substrate, the sample is transferred to the substrate, so that micro spots are formed on the substrate. Moreover, a dual spring structure including springs is used to transfer the sample, reducing the damage of the pins and substrate. In this case, only one process of spotting is basically achieved by one process of reserving the samples. This also causes a problem regarding the productivity.
In these conventional methods for forming micro spots, the sample solution is transferred onto the substrate in the state of exposing it to the atmosphere. Accordingly, the sample is dried in the course of transmission, thereby making it impossible to form the spots. This causes a problem regarding the efficiency in using a very expensive sample solution.
In order to overcome the above-mentioned problems resided in the methods for producing micro spots, a noncontact spotting method can be employed. As a device for dispensing a very small amount of a biomaterial with a high precision using the method, a micropipette wherein the piezoelectric/electrostrictive element is used as a micropump and a dispenser in which the micropipettes used are developed and put to practical use.
In the non-contact spotting method, a biomaterial containing a nucleic acid, an amino acid or the like is ejected as micro droplets into the air and adhered to a substrate of a slide plate, so that the above-mentioned problems in the methods, where the tip of a pin is in contact with the substrate, can be overcome.
In this method, however, a biomaterial having a relatively high viscosity is ejected as micro droplets into the air and then adhered to the substrate of the slide plate. Consequently, in addition to desired droplets (desired ejected droplets), so-called satellites (spray-like droplets being smaller than the desired ejected droplets) are produced and then adhered to the substrate. This causes problems regarding the quality in the products to be obtained, for instance, the generation of spots at positions other than the aimed positions of the desired spots, the disturbance in a fixed spacing between the adjacent micro spots, the generation of contamination due to the mutual mixture of the micro spots, etc. In some cases, such satellites do not generate at the initial stage of operating the dispenser, but generate after a while. This provides a very difficult problem in the production process control.
When, moreover, the ejecting velocity of the droplets increases, the energy of the droplets increases just at the moment at which the droplets are adhered to the substrate of the slide plate, and therefore sprays (splashes) are generated. Hence, this causes a problem that undesired spots (this can also be mentioned as satellites) resulting from the splashes are generated around desired spots. A decrease in the velocity of ejecting may suppress the generation of satellites. When, however, the velocity of ejecting is decreased, there occurs a problem that the ejection becomes unstable.
In order to form spots with a high density, it is necessary to always stabilize the ejection of droplets in a constant (straight) direction. For this purpose, it is principally possible to reduce the scattering in the direction of ejection by decreasing the spacing between the substrate of the slide plate and the nozzles for ejection. However, this causes such a problem that the decrease of the spacing becomes impossible because the slide plate itself includes varied positional thickness and, adding thereto, it is impossible to avoid the increase of cost, if the mechanical accuracy of the spotting machine itself must be improved.
Moreover, the biomaterial containing, e.g., DNA or the like normally has a greater viscosity, so that, after ejection and deposition, it is necessary to quickly dry it so as not to extend the spots on the substrate of the slide plate. In the case of using such a sample, there are problems that the nozzle for ejection is prone to be dried and the nozzle is prone to be clogged due to the increased viscosity of the sample, thereby making it impossible to eject the sample therefrom.
On the other hand, the spotting method employing an ink jet method in a printer has been investigated. Such an ink jet recording head has been disclosed in, for instance, JP-A-59-178258, wherein the opening of the nozzle for ejecting the ink has at least one angular corner and the capillary force resulting from the corner is used.
Although the head disclosed in the publication has an appreciated effect of preventing the bubbles from entering the nozzle, no such a satisfactory result as mentioned above can be obtained regarding the suppression of the generation of the so-called satellites or the like.
An ink jet head having a symmetric 2n-corner polygon shape of the opening for ejecting the sample (n is an integral number greater than 3) and having an ink channel, whose cross section in the direction perpendicular to the ejecting direction is trapezoidal, has been disclosed in Japanese Unexamined Patent Application Publication No. 3-101960.
Although the head disclosed in this publication also provides a prominent effect regarding both the required amount of ink droplets and the ejecting velocity thereof, no such a satisfactory result as mentioned above can be obtained to suppress the generation of the so-called satellites or the like.
Since, moreover, the subject matter of the inventions disclosed in these publications are not mainly directed to form the spots of biomaterials, but to form the spots of an ink material, it is difficult to directly apply the techniques disclosed in these publications to the present invention. Namely, there is a serious problem from the viewpoint of the size and cost if several thousands to several tens of thousands of separate channels are disposed in such an ink jet recording head. In the ink jet method, moreover, there is a problem from the viewpoint of the efficiency in usage of the sample, since it is necessary for ink jet recording heads to fill the pumps with the sample prior to spotting without generation of bubbles, so that it is necessary to use a greater amount of samples for purge. In general, it is very effective, if a liquid can move in a channel including a pump chamber with a high speed. In the case of using a delicate DNA solution, however, there is a problem regarding the damage of the DNA or the like due to the stirring of the sample in the channels.